New strategy to incorporate nano-particle sized water oxidation catalyst into dye-sensitized photoelectrochemical cell for water splitting

In order to develop a new strategy to deposit nano-particle sized water oxidation catalyst based on earth abundant element to the photoanode in a photoelectrochemical cell for water splitting, Co;O;as water oxidation catalyst was prepared and subsequently modified by 3-aminopropyltriethoxysilane. The amino functionalized Co;O;catalyst was carefully characterized and then integrated to the ruthenium dye sensitized photoelectrode through fast Schiff base reaction. Cyclic voltammetry experiments in the dark confirmed that the modified Co;O;catalyst was still active toward water oxidation, which could be initiated by oxidation of the ruthenium photosensitizer. Under visible light irradiation, incorporation of the modified Co;O;catalyst resulted in dramatic enhancement of the transient photocurrent density for the photoanode, which was 8 times higher than that of without Co;O;catalyst.

High-performance photoelectrochemical cells with MoS2 nanoflakes/TiO2 photoanode on 3D porous carbon spun fabric

A solar-driven photoelectrochemical(PEC)cell is emerging as one of the promising clean hydrogen generation systems.Engineering of semiconductor heterojunctions and surface morphologies of photoelectrodes in a PEC cell has been a primitive approach to boost its performance.This study presents that a molybdenum disulfide(MoS_(2))nanoflakes photoanode on 3-dimensional(3D)porous carbon spun fabric(CSF)as a substrate effectively enhances hydrogen generations due to sufficiently enlarged surface area.MoS_(2)is grown on CSFs utilizing a hydrothermal method.Among three different MoS_(2)coating morphologies depending on the amount of MoS_(2)precursor and hydrothermal growth time,film shape MoS_(2)on CSFs had the largest surface area,exhibiting the highest photocurrent density of 26.48 mA/cm^(2)and the highest applied bias photon-to-current efficiency(ABPE)efficiency of 5.32%at 0.43 VRHE.Furthermore,with a two-step growth method of sputtering and a subsequent hydrothermal coating,continuous TiO_(2)/MoS_(20 heterojunctions on a porous CSF further promoted the photoelectrochemical performances due to their optimized bandgap alignments.Enlarged surface area,enhanced charge transfer,and utilization of visible light enable a highly efficient MoS_(2)/TiO_(2)/CSF photoanode with a photocurrent density of 33.81 mA/cm^(2)and an ABPE of 6.97%at 0.87 VRHE.The hydrogen generation amount of the PEC cell with MoS_(2)/TiO_(2)/CSF photoanode is 225.4μmol/L after light irradiation of 60 s.

CdS quantum dot sensitized p-type NiO as photocathode with integrated cobaloxime in photoelectrochemical cell for water splitting

CdS sensitized NiO electrode was used as the photoactive cathode in a photoelectrochemical cell for water splitting,avoiding the use of a sacrificial electron donor.Photocurrent increment under visible light irradiation was observed after integration of[Co（dmgH）_2（4-Me-py）Cl]（1） to the photocathode,suggesting 1 could accept electrons from photoexcited CdS for water reduction and NiO could move the holes in the valence band of CdS to anode for water oxidation.

Application of Novel Calix[4]arene Metal-free Sensitizers in Dye-sensitized Photoelectrochemical Cells for Water Splitting

A series of novel calix[4]arene metal-free dyes,featuring macrocyclic structure and unique conical confor mation,has been introduced into photoanode-based dye-sensitized electrochemical cell system as photosensitizers.The electrochemical properties of the corresponding sensitized photoanodes were systematically studied in the absence.presence of water oxidation catalyst(WOC).Furthermore,the visible-light-driven overall water-splitting reactions wero conducted by fully assembled devices,obtaining a performance trend of Calix-3>Calix-2 Calix-1.The correspondins device of Calix-3 exhibited the best photoactivity,giving an initial photocurrent density of ca.300 pA/cm^2,an IPEC peak value of ca.9.0%at 365 nm and a wide photo-respond band up to ca.620 nm.The best performance of Calix-3 an be attributed to its most effective light-harvesting ability,best ICT transition property,highest oxidation potentiaand thus best ability of activating WOC.This work offers an inspiration for the application of new-type effective metal-free sensitizers in photocatalytic water-splitting device.

Chlorophyll a photoelectrochemical cells with different metal electrodes

A photosensitive electrode was prepared by electrodepositing a membrane of chlorophyll a (Chla) on a SnO2 optical transparent electrode, with which and a metal counter electrode a Chla photoelectrochemical cell was formed. Photoinduced current (I-i) and photoinduced voltage (V-i) of the cell were measured. The dependence of I-i on the properties of metal electrodes was obvious, which was illustrated with mechanism bf Chla photoelectrical effects. I-i in this work was as high as 2x10(-5) A . cm(-2).

Photo-driven water splitting photoelectrochemical cells by tandem organic dye sensitized solar cells with I-/I3- as redox mediator

Dye-sensitized photoelectrochemical tandem cells have shown the promise for light driven hydrogen production from water owing to the low cost,wide absorption spectra in the visible region and ease to process of their constitutive photoelectrode materials.However,most photo-driven water splitting photoelectrochemical cells driven by organic dye sensitized solar cells exhibit unsatisfactory hydrogen evolution rate,primarily attributed to their poor light capturing ability and low photocurrent performance.Here we present the construction of a tandem system consisting of an organic blue-colored S5 sensitizer-based dyesensitized photoelectrochemical cell(DSPEC) wired in series with three spectral-complemental dyes BTA-2,APP-3 and APP-1 sensitizers-based dye-sensitized solar cell(DSC),respectively.The two spectral-complemental chromophores were used in DSC and DSPEC to ensure that the full solar spectrum could be absorbed as much as possible.The results showed that the photocurrent of tandem device was closely related to the open-circuit voltage(Voc) of sensitized DSC,in which the tandem configuration consisting of S5 based DSPEC and BTA-2 based DSC gave the best photocurrent.On this basis,tandem device with the only light energy and no external applied electrical bias was further constructed of BTA-2 based 2-junction DSC and S5 based DSPEC and obtained a photocurrent of 500 μA cm-2 for hydrogen generation.Furthermore,I-/I3-was used as a redox couple between dye regeneration and O2 production on the surface of Pt-IrO2/WO3.The strategy opens up the application of pure organic dyes in DSC/DSPEC tandem device.

Improvement of electrical and photovoltaic properties of methyl red dye based photoelectrochemical cells in presence of single walled carbon nanotubes

In this work, we investigated the effect of single walled carbon nanotubes （SWCNT） on the electrical and photovoltaic properties of methyl red （MR） dye based photoelectrochemical cell （PEC）. MR dye based PEC with LiCl04 as ion salt were fabricated with and without mixing SWCNT. The cells were characterized through electrical and optical measurements. The performance of the devices changed drastically in presence of SWCNT. The transition voltage and trap energy of the cells were estimated from the steady-state dark current voltage （I-V） analysis. The transition voltage and trap energy decreased for MR dye cell in presence of SWCNT. Open circuit voltage （Voc）, short circuit current （Jsc）, fill factor （FF） and power conversion efficiency （v/） increased due to the addition of SWCNT. Further measurement of the transient photo- current showed that the growth and decay of photocurrent was quite faster in presence of SWCNT. The photocurrent decay with time was fitted for both the cells and found to follow a power law relation which indicates dispersive transport mechanism with exponential trap states distrib- uted in between lowest unoccupied molecular orbital （LUMO） and highest occupied molecular orbital （HOMO） levels. Possible interpretation is done on the lowering of trap energy with the photocurrent. These results suggest that SWCNT lowers the trap energy of the cells by providing efficient percolation pathways for the conduction of charges. It is expected that due to lowering of trap energy the residing time of the free carriers within the traps decreases. In other words, it may also be said that the charge recombination decreases. These factors affect the overall conduction of charges and improve the electrical and photovoltaic properties.

Influence of O-O formation pathways and charge transfer mediator on lipid bilayer membrane-like photoanodes for water oxidation

Inspired by the function of crucial components in photosystemⅡ(PSⅡ),electrochemical and dyesensitized photoelectrochemical(DSPEC)water oxidation devices were constructed by the selfassembly of well-designed amphipathic Ru(bda)-based catalysts(bda=2,2'-bipyrdine-6,6'-dicarbonoxyl acid)and aliphatic chain decorated electrode surfaces,forming lipid bilayer membrane(LBM)-like structures.The Ru(bda)catalysts on electrode-supported LBM films demonstrated remarkable water oxidation performance with different O-O formation mechanisms.However,compared to the slow charge transfer process,the O-O formation pathways did not determine the PEC water oxidation efficiency of the dyesensitized photoanodes,and the different reaction rates for similar catalysts with different catalytic paths did not determine the PEC performance of the DSPECs.Instead,charge transfer plays a decisive role in the PEC water oxidation rate.When an indolo[3,2-b]carbazole derivative was introduced between the Ru(bda)catalysts and aliphatic chain-modified photosensitizer in LBM films,serving as a charge transfer mediator for the tyrosine-histidine pair in PSⅡ,the PEC water oxidation performance of the corresponding photoanodes was dramatically enhanced.

Photoelectrochemical study of MoO_3 assorted morphology films formed by thermal evaporation

Molybdenum oxide nanostructured thin films were grown on fluorine doped tin oxide（FTO）, indium doped tin oxide（ITO） and ordinary glass substrates by thermal evaporation process without vacuum and catalysts using molybdenum trioxide（MoO） powder as a source material and oxygen as a carrier gas.Various morphologies including nanobelts, disks and hexagonal rod-like nanostructures were obtained by changing the source and substrate temperatures during the growth of MoOthin films. Structural parameters, morphology, composition and surface features of the films were characterized by XRD, SEM, EDAX,XPS, AFM and Raman spectroscopy. The films were orthorhombic in structure with preferred orientation along（0 1 0） plane. Morphology analysis reveals randomly aligned nanobelts with 40 nm in thickness and a width of 800 nm and 3–12 mm in length. The disks have 1.5 μm diameters, 1 μm thickness and hexagonal rod-like nanostructures with a length, breath and width of 2 μm, 1 μm and 100 nm are formed. The samples were investigated under dark and photocurrent conditions in HSOaqueous solution as a function of applied potential. The photocurrent density of samples prepared on ITO and FTO substrate samples were compared and the results are discussed.

Analysis of the factors controlling performances of Au-modified TiO2 nanotube array based photoanode in photo-electrocatalytic(PECa)cells

The efficiency of photo-electrocatalytic（PECa） devices for the production of solar fuels depends on several limiting factors such as light harvesting, charge recombination and mass transport diffusion. We analyse here how they influence the performances in PECa cells having a photo-anode based on Au-modified TiOnanotube（TNT） arrays, with the aim of developing design criteria to optimize the photo-anode and the PECa cell configuration for water photo-electrolysis（splitting） and ethanol photo-reforming processes.The TNT samples were prepared by controlled anodic oxidation of Ti foils and then decorated with gold nanoparticles using different techniques to enhance the visible light response through heterojunction and plasmonic effects. The activity tests were made in a gas-phase reactor, as well as in a PECa cell without applied bias. Results were analysed in terms of photo-generated current, Hproduction rate and photoconversion efficiency. Particularly, a solar-to-hydrogen efficiency of 0.83% and a Faradaic efficiency of 91%were obtained without adding sacrificial reagents.

Unassisted overall water splitting with a solar-to-hydrogen efficiency of over 10% by coupled lead halide perovskite photoelectrodes

Hydrogen is a promising future sustainable fuel candidate with boundless opportunities.Research into photoelectrochemical(PEC)water splitting based on a lead halide perovskite(LHP)has progressed significantly with the aim of more efficient solar hydrogen production.Herein,we unite a well-known photo-absorbing LHP with cost-effective water-splitting catalysts,and we introduce two types of monolithic LHP-based PEC devices that act as a photocathode and a photoanode for the hydrogen evolution reaction and oxygen evolution reaction,leading to efficient unbiased overall water splitting.Through the integration of these two monolithic LHP-based photoelectrodes,an unbiased solar-to-hydrogen conversion efficiency of 10.64%and a photocurrent density of 8.65 mA cm^(−2) are achieved.

Surface treatment of GaN nanowires for enhanced photoelectrochemical water-splitting

High-efficiency hydrogen production through photoelectrochemical(PEC)water splitting has emerged as a promising solution to address current global energy challenges.Ⅲ-nitride semiconductor photoelectrodes with nanostructures have demonstrated great potential in the near future due to their high light absorption,tunable direct band gap,and strong physicochemical stability.However,several issues,including surface trapping centers,surface Fermi level pinning,and surface band bending,need to be addressed.In this work,enhanced photovoltaic properties have been achieved using gallium nitride(GaN)nanowires(NWs)photoelectrodes by adopting an alkaline solution surface treatment method to reduce the surface states.It was found that surface oxides on NWs can be removed by an alkaline solution treatment without changing the surface morphology through X-ray photoelectron spectroscopy(XPS),scanning electron microscopy(SEM)and other characterization methods.These findings provide new insights to the development of high-efficiency photoelectrodes for new energy source applications.

Artificial photosynthesis for high-value-added chemicals:Old material,new opportunity

Solar energy utilization has drawn attention due to ever-increasing environmental and energy issues.Photoelectrochemical(PEC)and photocatalytic(PC)water splitting for hydrogen production,which is the most popular and well-established solar-to-chemical conversion process,has been studied thoroughly to date but is now facing limitations related to low conversion efficiency.To resolve this issue,research in PEC cells or photocatalysts has recently aimed to produce alternative value-added chemicals by modifying their redox reactions,which potentially enables high economic reward to compensate for the low efficiency.Here,various kinds of redox reactions that decouple classic water splitting reactions to produce value-added chemicals via PEC and PC processes are introduced.Successful coupling of CO_(2) reduction,O_(2) reduction and organic synthesis with either water oxidation or water reduction is comprehensively discussed from the perspective of basic fundamental and product selectivity in terms of the band structure of materials,cocatalyst design,and thermodynamics and kinetics of the reactions.Throughout the review,future challenges and opportunities are suggested with respect to the redesigned artificial synthesis,which might be an alternative development for the commercialization of PEC or PC value-added chemical production technologies in the near future.

Nanocrystalline Iron Pyrophosphate-Regulated Amorphous Phosphate Overlayer for Enhancing Solar Water Oxidation

A rational regulation of the solar water splitting reaction pathway by adjusting the surface composition and phase structure of catalysts is a substantial approach to ameliorate the sluggish reaction kinetics and improve the energy conversion efficiency.In this study,we demonstrate a nanocrystalline iron pyrophosphate(Fe_(4)(P_(2)O_(7))_(3),FePy)-regulated hybrid overlayer with amorphous iron phosphate(FePO_(4),FePi)on the surface of metal oxide nanostructure with boosted photoelectrochemical(PEC)water oxidation.By manipulating the facile electrochemical surface treatment followed by the phosphating process,nanocrystalline FePy is localized in the FePi amorphous overlayer to form a heterogeneous hybrid structure.The FePy-regulated hybrid overlayer(FePy@FePi)results in significantly enhanced PEC performance with long-term durability.Compared with the homogeneous FePi amorphous overlayer,FePy@FePi can improve the charge transfer efficiency more significantly,from 60% of FePi to 79%of FePy@FePi.Our density-functional theory calculations reveal that the coexistence of FePi and FePy phases on the surface of metal oxide results in much better oxygen evolution reaction kinetics,where the FePi was found to have a typical down-hill reaction for the conversion from OH*to O_(2),while FePy has a low free energy for the formation of OH*.

Highly Efficient Photosensitization of Mesoporous TiO_2 Electrode with a Cyanine Dye

A low cost cyanine dye, 1,1-dimethyl-3-ethyl-2-[3-(1,3-dihydro-3,3-dimethyl-1-ethyl- 5-carboxyl-2H-indol-2-ylidene)-1-propenyl]-1H-benz[e] indolium iodide (1) was synthesized and applied to sensitize mesoporous TiO2 electrode. Photoresponse of the electrode was extended to the visible and remarkably high incident photon-to-current conversion efficiency (IPCE) over 70% was achieved from 500 nm to 600 nm.

Dye-sensitized photoanode decorated with pyridine additives for efficient solar water oxidation

Splitting water into hydrogen and oxygen by dye-sensitized photoelectrochemical cell(DSPEC)is a promising approach to solar fuels production.In this study,a series of pyridine derivatives as surface additives were modified on a molecular chromophore and water oxidation catalyst co-loaded TiO_(2)photoanode,TiO_(2)|RuP,1(RuP=Ru(4,4′-(PO3H2)2-2,2′-bipyridine)(2,2′-bipyridine)2,1=Ru(bda)(L)2,(bda=2,2′-bipyridine-6,6′-dicarboxylate,L=10-(pyridin-4-yloxy)decyl)phosphonic acid).The addition of pyridine additives was found to result in up to 42%increase in photocurrent.Under simulated sun-light irradiation,TiO_(2)|RuP,1,P1(P1=4-Hydroxypyridine)produced a photocurrent density of 1 mA/cm2 at a bias of 0.4 V vs.NHE in acetate buffer.Moreover,the observed photocurrents are correlated with the electron-donating ability of the substituent groups on pyridine ring.Transient absorption measurements and electrochemical impedance spectroscopy revealed that surface-bound pyridine can effectively retard the back-electron transfer from the TiO_(2)conduction band to the oxidized dye,which is a major process responsible for energy loss in DSPECs.

Electrochemical creation of surface charge transfer channels on photoanodes for efficient solar water splitting

Electrochemical treatment is a popular and efficient method for improving the photoelectrochemical performance of water‐splitting photoelectrodes.In our previous study,the electrochemical activation of Mo‐doped BiVO_(4) electrodes was ascribed to the removal of MoO_(x) segregations,which are considered to be surface recombination centers for photoinduced electrons and holes.However,this proposed mechanism cannot explain why activated Mo‐doped BiVO_(4) electrodes gradually lose their activity when exposed to air.In this study,based on various characterizations,it is suggested that electrochemical treatment not only removes partial MoO_(x) segregations but also initiates the formation of H_(y)MoO_(x) surface defects,which provide charge transfer channels for photogenerated holes.The charge separation of the Mo‐doped BiVO_(4) electrode was significantly enhanced by these charge transfer channels.This study offers a new insight into the electrochemical activation of Mo‐doped BiVO_(4) photoanodes,and the new concept of surface charge transfer channels,a long overlooked factor,will be valuable for the development of other(photo)electrocatalytic systems.

High-performance bulk heterojunction-based photocathode with facile architecture for photoelectrochemical water splitting

Organic semiconductors are promising candidates as photoactive layers for photoelectrodes used in photoelectrochemical(PEC)cells due to their excellent light absorption and efficient charge transport properties with the help of interfacial materials.However,the use of multilayers will make the charge transfer mechanism more complicated and decrease the PEC performance of the photoelectrode caused by the increased contact resistance.In this work,a PM6:Y6 bulk heterojunction(BHJ)-based photocathode is fabricated for efficient PEC hydrogen evolution reaction(HER)in an acidic aqueous solution.With RuO_(2)as an interfacial modification layer,the photocathode with a simple structure(fluorine-doped tin oxide(FTO)/PM6:Y6/RuO_(2))generates a maximum photocurrent density up to-15 m A/cm^(2)at 0 V vs.reference hydrogen electrode(RHE),outperforming all previously reported BHJ-based photocathodes in terms of PEC performance.The highest ratiometric power-saved efficiency of 3.7%is achieved at 0.4 V vs.RHE.

Comparative architecture in monolithic perovskite/silicon tandem solar cells

Inorganic-organic metal halide perovskite light harvester-based perovskite solar cells(PSCs)with widely tunable bandgap have achieved rapid growth in power conversion efficiency,which exceeds 25%now.It is deliberated that if a semitransparent solar cell made of wider bandgap materials was placed on top of a narrow bandgap materials-based solar cell such as a silicon solar cell,with proper optical and electrical arrangements,the resultant tandem device consisting of two subcells could more effectively utilize the solar spectrum than a single junction solar cell.In a perovskite/silicon tandem solar cell(PSTSC),a semitransparent PSC with a wider bandgap is placed on top of a narrow bandgap silicon solar cell.The PSC efficiently harvests the higher energy photons in the ultraviolet and visible regions of the solar spectrum while the silicon solar cell can convert the photons of the infrared region to power.The PSTSC is proposed as a potential candidate to overcome the Shockley-Queisser limit of single-junction silicon solar cells.Though the theoretical limit of a PSTSC is calculated as~42%,its actual efficiency achieved until now is less than 30%.Therefore,a great scope of research exists in improving the efficiency of PSTSCs.Current issues of stability and upscaling of the device in PSCs are also a matter of concern for PSTSCs.A tandem device consists of multiple parts,and different configurations can be applied,thus tuning the architecture of the device.Altering various parts may result in significant changes in the efficiency of the device.In this review,competing architectures of otherwise comparable devices are compared in terms of photovoltaic properties.Thus,future directions to improve the efficiency of the device based on architecture design are proposed herein.In particular,the influence of the polarity of PSCs and the surface morphology of silicon solar cells(both front and rear)on determining the properties of the PSTSC are discussed.

Recent Advancements in Photoelectrochemical Water Splitting for Hydrogen Production

Sunlight is the most abundant and inexhaustible energy source on earth.However,its low energy density,dispersibility and intermittent nature make its direct utilization with industrial relevance challenging,suggesting that converting sunlight into chemical energy and storing it is a valuable measure to achieve global sustainable development.Carbon–neutral,clean and secondary pollution-free solar-driven water splitting to produce hydrogen is one of the most attractive avenues among all the current options and is expected to realize the transformation from dependence on fossil fuels to zero-pollution hydrogen.Artificial photosynthetic systems(APSs)based on photoelectrochemical(PEC)devices appear to be an ideal avenue to efficiently achieve solar-to-hydrogen conversion.In this review,we comprehensively highlight the recent developments in photocathodes,including architectures,semiconductor photoabsorbers and performance optimization strategies.In particular,frontier research cases of organic semiconductors,dye sensitization and surface grafted molecular catalysts applied to APSs based on frontier(molecular)orbital theory and semiconductor energy band theory are discussed.Moreover,research advances in typical photoelectrodes with the metal–insulator–semiconductor(MIS)architecture based on quantum tunnelling are also introduced.Finally,we discuss the benchmarks and protocols for designing integrated tandem photoelectrodes and PEC systems that conform to the solar spectrum to achieve high-efficiency and cost-effective solar-to-hydrogen conversion at an industrial scale in the near future.